The present invention relates to a microwave cooking oven and more specifically to an improved excitation system for such an oven which enhances the time-averaged uniformity of energy distribution within the cooking cavity.
A continuing problem in the design of microwave cooking ovens is to eliminate hot and cold spots in the cooking cavity resulting from the non-uniform spatial distribution of energy in the cavity. Such non-uniform energy distribution is often explained to be the result of the establishment of electromagnetic standing wave patterns, known as "modes," within the cooking cavity. When such standing wave patterns exist in the cavity, the intensity of the electric and magnetic fields vary greatly with position. The particular mode patterns which may be established in the cavity are dependent upon many variables including the frequency of the microwave energy used to excite the cavity and the dimensions of the cavity.
A number of different approaches to enhance uniform energy distribution by altering the standing wave patterns in the cavity have been tried. One common approach involves the use of a so-called "mode stirrer" which typically resembles a fan with metal blades. This stirrer is normally located near the point where energy is coupled into the cooking cavity, such as in the cooking cavity itself or in the waveguide, coupling energy from the magentron to the cavity near an exit port of the waveguide. In any case, the mode stirring approach is an attempt to randomize energy reflections in the cavity by introducing time varying scattering of the microwave energy by reflection from the stirrer blades as the microwave energy enters the cavity. While mode stirring has been found to provide some improvement in energy distribution uniformity, side-to-side and front-to-back field strength variations are not entirely eliminated.
Another approach has involved the use of a rotating antenna in the cavity. Prior art relating to such use of rotating antenna may be found in U.S. Pat. No. 4,028,521 to Uyeda et al; 4,284,868 to Simpson; and 4,316,069 to Fitzmayer, for example. Even though rotating antennas tend to improve uniformity of energy distribution in the cavity, typical antenna configurations tend to leave cold spots. For centrally mounted antenna, such cold spots tend to occur near the center of rotation of the antenna. Rotating antenna arrangements also have a significant assembly disadvantage in that energy coupling efficiency and impedance matching are extremely sensitive to assembly tolerances. For example, coupling efficiency is extremely sensitive to the depth of penetration of the antenna probe into the waveguide; also, antenna impedance is extremely sensitive to the spacing between the antenna arms and the ground plane; i.e., the adjacent cavity wall. In addition, antennas generally protrude into the cooking cavity, reducing the usable cavity space.
The use of radiating slots is also known in the art. U.S. Pat. Nos. 4,019,009 to Kusunoki et al; U.S. Pat. No. 2,804,802 to Blass et al; and U.S. Pat. No. 3,810,248 to Risman et al provide examples of stationary radiating slots arranged beneath the food load to be heated. U.S. Pat. No. 3,210,511 to Smith provides single diametrically opposed slots on the top and bottom walls of the cooking cavity oriented at right angles to each other to produce circularly polarized radiation in the cavity.
U.S. Pat. No. 4,327,266 to Austin et al combines a rotating antenna and slot to provide a coaxially fed bilaterally symmetrical rotating plate antenna disposed near the bottom wall of the cavity and having radiating wings at its periphery and a substantially tangential radiating slot closely adjacent its axis of rotation, which purportedly results in uniform microwave heating of food items in the cavity due to a balance between aperture radiation and wing radiation. This antenna configuration appears to protrude into the cavity to an undesirable extent.
U.S. Pat. No. 3,746,823 seeks to provide improved energy distribution uniformity by providing a rotating disk having formed therein several elongated radiating apertures sequentially oriented transverse to the longitudinal waveguide axis, each aperture being oriented and positioned such that when transverse to the longitudinal waveguide axis the apertures appear electrically at integral multiples of half-wave points from the magnetron to achieve maximum energy transfer through the transverse apertures, while allowing only a minimum amount of energy to be transferred into the cavity when the apertures are aligned parallel to the longitudinal waveguide axis, thus appearing to produce a radiation system permitting maximum power transfer to the cavity. However, such an arrangement is believed to be limited as to time-averaged uniformity of energy distribution in the cavity.
While each of the approaches mentioned herein appears to provide some improvement in the attempt to overcome the energy non-uniformity problem in microwave ovens, a need remains for a relatively simple, efficient low profile energization system which provides good uniformity of energy distribution in the cooking cavity without extending obtrusively into the cavity so as to maximize the space available in the cavity to receive items to be heated.
It is therefore an object of the present invention to provide a relatively simple, efficient excitation system for a microwave oven which enhances the time-averaged uniformity of energy distribution within the cavity employing an extremely low profile radiating member which projects only minimally into the cooking cavity.